1. Field of the Invention
The present invention relates to an endoscope including a device for adjusting an illuminance ratio between moving and still picture images.
2. Description of the Background Art
In a conventional endoscope, when picture images are picked up by using continuous light, reproduced picture images are blurred by a freezing operation, and the resolving power of the reproduced picture images is extremely reduced. In order to solve this problem, a pulsed light is used in the freezing operation. That is, in this case, when a moving picture image pickup is carried out, continuous lighting is performed by applying a DC current to a light source, and a time width of one frame is coincident with a radiation time width of the same. In turn, when a still picture image pickup is carried out in response to a freezing operation, a light pulse extending over the boundary between adjacent odd and even fields is emitted by a light source every one frame.
In this case, when the moving and still picture images are picked up and the illuminance of the moving picture image is equal to that of the still picture image, there is no difference in brightness between the moving and still picture images on a display, and thus it is very convenient and easy for an operator to observe the displayed picture images.
In order to adjust the luminous energy or illuminance ratio between the moving and still picture images to be the same or close to each other, one of predetermined values of a tube current to be applied to a lighting tube such as a xenon lamp of a light source is elected to change the light intensity of the light source. That is, in this case, the light intensity of the light source during pickup of the moving picture image is determined by selecting one of the fixed tube current values.
However, after the lighting tube of the light source has been used for a long period of time, its light output becomes gradually lowered, and thus a change arises in the light intensity ratio initially determined at an entrance end of a light guide of a scope between the continuous DC current lighting for picking up the moving picture image and the pulsed lighting for picking up the still picture image in the light source. Hence, a change in brightness between the moving and still picture images appears on the monitor, and the difference in brightness between the moving and still picture images is gradually increased.